In the present GSM (Global System for Mobile communications) system, the network informs from time to time each mobile station about the base stations nearby the mobile station. The network performs this informing by sending a so called neighbor list to the mobile station, which neighbor list recites the basic parameters of the neighboring cells, such as the base station identity code (BSIC) of the base stations of the cells, the BCCH (Broadcast Control Channel) frequency, and several other parameters.
FIG. 1 illustrates a part of the structure of a GSM network. FIG. 1 shows a mobile station (MS) 10, base stations (BS) 20, and base station controllers (BSC) 30. For clarity, areas of the cells corresponding to the base stations and other network elements of a GSM network are not illustrated in FIG. 1. The neighboring cells are simply the cells lying nearby the mobile station. One possible selection of neighboring cells is marked with a rectangle 25 in FIG. 1.
The network 20, 30 informs the mobile station 10 about the neighboring cells in a message, which recites the cell parameters for each neighboring cell. The resulting message may be long, if the number of neighboring cells is large. According to the present GSM specifications, the maximum number of cells reported in a neighbor cell information message is 16. Due to the length of the message, the transmission time required to transmit the message is long, which results in a relatively large usage of signaling channel capacity for only this purpose. Also, during the transmission of the message the receiver of the mobile station receiving the message has to receive the message, whereby the length of the message affects considerably the receiving capacity of the mobile station. The length of the message also makes it impractical to increase the number of cells described in a neighbor cell information message, which would be advantageous especially in those locations of the network, in which a large number of cells are available to a mobile station or may become available to the mobile station due to movement of the mobile station. If the repetition frequency of sending the message would be decreased, the length of the message could be increased, but in that case, the mobile stations would receive the message less often, which could cause other difficulties. These problems are presently increasing in severity due to the present trend in the cellular telecommunication systems towards decreasing the area of cells due to increasing data rates, whereby the increasing of the number of cells described in a neighbor cell information message is desired. This trend is driven by the development of high data rate services and packet data services, such as the GPRS (General Packet Radio Service) system.
The transmitted parameters and structure of a neighbor cell information message is described in more detail in the GSM 04.08 specification.
An object of the invention is to realize a method which allows shortening the time required for the communication of a neighbor cell information message. An object of the invention is also to realize a method of communicating neighbor cell information reserving the receiver of a mobile station for a shorter time than in the prior art. A further object of the invention is to save the transmission capacity of signaling channels used for transmitting neighbor cell information messages.
The objects are reached by identifying redundant information in the neighbor cell information message, and removing at least a part of the redundancy by specifying the repeating values of at least one parameter only once.
According to the invention, a list of neighbor cell information is communicated to a mobile station in compressed form. Preferably, the neighbor cell information list is transmitted in such a way, that a table reciting parameter values in use by the neighboring cells, and for each of these cells, each value listed in the table is represented by a pointer such as an index to the table. In this way, same parameter values do not need to be repeated for each cell using the same values. The neighbor cell information list can be further compressed by expressing a first frequency parameter value in the normal way, but expressing further frequency parameter values relative to the first, or as in a further embodiment, relative to the previous frequency parameter value. Such ways of representing frequency values allow the use of fewer bits to represent the frequency values.
The inventive method of constructing a neighbor cell information message is advantageous, since the inventors have made the insightful observation, that neighboring base stations quite commonly have largely similar operation parameters, which results in the repeating of same parameter values for a plurality of cells in a neighbor cell information message. The length of a neighbor cell information message, and time and transmission capacity required for sending such a message can be considerably shortened by removing at least some of the redundancy of information in the neighbor cell information message by specifying at least some repetitive values only once, and replacing the occurrences of the value with a pointer to the single specification of the value. The inventive method of constructing a neighbor cell information message also allows the describing of more cells in a neighbor cell information message than in prior art, since the compressed message format allows the increasing of the number of base station described in the message without unduly increasing the length of the message.